1. Field of the Invention
The present invention relates to a PCM (Pulse Code Modulation) recording and reproducing apparatus and more particular to error detecting and error compensating circuits in the reproducing part of a PCM recording and reproducing apparatus in which the digital information is recorded in a frame distribution system on the recording medium such as a magnetic tape in the running direction of the recording medium.
2. Description of the Prior Art
A PCM recording and reproducing apparatus has hitherto been known as a typical application of the digital recording technique. Such an apparatus is classified into two types: One is of the type using a rotary head mechanism as in the case of the magnetic video tape recorder (VTR); the other is of the type using a fixed head.
In the VTR, the relative velocity of the moving tape to the rotating head is very high, for example, the relative velocity ranging from 10 m/sec to 40 m/sec. Accordingly, this tape of apparatus is suitable for a recording system requiring a wide frequency band width such as a PCM recording system, and is advantageous in that it may provide an apparatus being stable against noise with a high redundancy. However, it has a problem in that the construction is complex and the manufacturing cost is high, since it inherently requires some parts rotating at a high speed and high accuracy.
On the other hand, in the system using a fixed head with multi-tracks, the recording of the information on the tape may be made with a wide frequency band width without any reduction of recording wave length. The reason for this is that an increase of the transmission rate of the pulses which is caused when the information signal is coded or quantized, is absorbed by dispersing it to the respective tracks on the tape. Moreover, in this system, an economical and simpler construction may be expected for a reason that there is no need of the mechanism operating at a high speed and high accuracy, unlike the VTR.
The PCM recorder using the fixed head with multi-track is classified into two types from the standpoint of distributing on the respective tracks the tape coded pulse trains. One is a bit distribution system and the other is a frame distribution system.
In the bit distribution system, a series of coded pulses are sequentially distributed from MSB (Most Significant Bit) to LSB (Least Significant Bit) on the respective tracks each arranged in parallel but normal to the running direction of the tape. Each interval between adjacent bits on a track of the tape corresponds to the period of the sampling frequency. For example, if the running velocity of the tape is taken as 38 cm/sec and the sampling frequency 40 kHz, then the bit interval on the track is 9.5 .mu.m.
A "dropout" which is a very small blank or defect on a magnetic tape produced in manufacturing process of the tape, is inevitable in magnetic tapes. If a dropout of about 100 .mu.m occurs on a track, approximate 10 samples of the information are sequentially dropped out when considering the above example.
In the frame distribution system, a train of pulses coded or quantized through an analogue to digital converter are grouped into a series of frames each corresponding to predetermined number of samples, and the frames in turn is sequentially distributed on the respective tracks in the tape. A series of incoming pulse trains each of one frame are reduced in the transmission rate in accordance with the number of the tracks on the magnetic tape and then the coded pulse trains are recorded on the tape along the respective tracks.
For example, if the running speed of the magnetic tape is taken as 38 cm/sec, the sampling frequency as 40 kHz, the bit number constituting one word as 12 bits, and the number of tracks is 12, then the bit interval of adjacent bits is in the order of 9.5 .mu.m, as in the case of the above bit distribution system. In this case, the recording length of one sample is 9.5 .mu.m .times. 12 and thus the dropout of about 100 .mu.m causes only one to two samples to be damaged.
Thus, the frame distribution system remarkably improves the adverse effect by the dropout over the bit distribution system. It is customary to use an error check bit for checking the recorded information to which errors may be introduced due to disturbance or the like when the digital information is recorded or reproduced on the recording medium. In the frame distribution system, one frame consists of a plurality of words each also consisting of a number of information pulses corresponding to one sample value and an error check bit located following the former pulse train of information.
The frame distribution system has a defect arising from the fact that an error check bit is located immediately after an information word pulse train corresponding to one sample value. That is, when the dropout occurs, the error check bit as well as the information word pulse train is damaged. This will be described below in more detail.
It is assumed that one word consists of 14 bits in which 12 bits are assigned to the data bits and 2 bits to the check bits. If a large dropout sufficiently covering one word occurs, the digital information to be reproduced generally becomes random in the bit arrangement. This disarrangement of course slightly depends on the methods of modulation and demodulation when the digital information is recorded and reproduced on and from the recording medium.
Under such a random information, even if any type of check system is employed, there is the probability of 1/4 of the information word having the error being extracted from the information recorded on the magnetic tape, so far as the check bit of 2 bits is used. In other words, there is a probability of 1/4 that the random information may be transferred as correct information to the output system of the recording apparatus.
For this, there has been proposed another system in which, in the case of deciding whether an information word at time t.sub.n is correct or not, its correctness is decided only when the word at time t.sub.n has no error and further several words before and after the word include no error. This proposed system is effective in the case where the signal error rate is low and the error check is directed to solely remove the problems resulting from the dropout. It is expected, however, that the signal error rate becomes high when the information-recording density is made high for reducing the size of the recording apparatus. Further, the quality of the sound reproduced will be deteriorated, since occurrence of the dropout and a single word as well, respectively, are considered to be the occurrence of errors in several words.